Virtual boundary processing simplification for adaptive loop filtering (ALF) in video coding

Abstract

Virtual boundary processing in adaptive loop filtering (ALF) requires that padded values be substituted for unavailable pixel rows outside the virtual boundaries. Methods and apparatus are provided for virtual boundary processing in ALF that allow the use of more actual pixel values for padding than in the prior art.

The invention claimed is: 
     
       1. A method comprising:
 determining filter coefficients for a two-dimensional (2D) finite impulse response (FIR) filter; 
 applying the 2D FIR filter using the determined filter coefficients to a first pixel p(x,y) at a first virtual boundary of a reconstructed coding unit, wherein x is a coordinate on an x-axis of the reconstructed coding unit and y is a coordinate on a y-axis of the reconstructed coding unit; 
 computing a first filtered pixel value based on the filtered first pixel p(x,y) and replicated values in which a replicated value for a pixel p(x−1,y−1) is replicated from a value of a pixel p(x−1,y); and 
 outputting the first filtered pixel value. 
 
     
     
       2. The method of  claim 1 , wherein determining filter coefficients comprises decoding the filter coefficients from an encoded bit stream. 
     
     
       3. The method of  claim 1 , wherein applying the 2D FIR filter to the first pixel comprises using filter coefficients determined for a neighboring largest coding unit (LCU) of the reconstructed coding unit to compute the first filtered pixel value. 
     
     
       4. The method of  claim 1 , wherein the 2D FIR filter is a 10-tap FIR filter with a vertical size of 7 and a horizontal size of 9. 
     
     
       5. A device comprising one or more processors configured to:
 determine filter coefficients for a two-dimensional (2D) finite impulse response (FIR) filter; 
 apply the 2D FIR filter using the determined filter coefficients to a first pixel p(x,y) at a first virtual boundary of a reconstructed coding unit, wherein x is a coordinate on an x-axis of the reconstructed coding unit and y is a coordinate on a y-axis of the reconstructed coding unit; 
 compute a first filtered pixel value based on the filtered first pixel p(x,y) and replicated values in which a replicated value for a pixel p(x−1,y−1) is replicated from a value of a pixel p(x−1,y); and 
 output the first filtered pixel value. 
 
     
     
       6. The device of  claim 5 , wherein the one or more processors are further configured to decode the filter coefficients from an encoded bit stream. 
     
     
       7. The device of  claim 5 , wherein the one or more processors are further configured to:
 apply the 2D FIR filter to the first pixel comprises using filter coefficients determined for a neighboring largest coding unit (LCU) of the reconstructed coding unit to compute the first filtered pixel value. 
 
     
     
       8. The device of  claim 5 , wherein the 2D FIR filter is a 10-tap FIR filter with a vertical size of 7 and a horizontal size of 9. 
     
     
       9. The method of  claim 1 , wherein computing the first filtered pixel value is based on the replicated values in which a replicated value for a pixel p(x+1,y+1) is replicated from a value of a pixel p(x+1,y). 
     
     
       10. The method of  claim 1 , wherein computing the first filtered pixel value is based on the replicated values in which a replicated value for a pixel p(x,y+2) is replicated from a value of the first pixel p(x,y). 
     
     
       11. The method of  claim 1 , wherein computing the first filtered pixel value is based on the replicated values in which a replicated value for a pixel p(x,y+3) is replicated from a value of the first pixel p(x,y). 
     
     
       12. The device of  claim 5 , wherein the one or more processors are configured to compute the first filtered pixel value based on the replicated values in which a replicated value for a pixel p(x+1,y+1) is replicated from a value of a pixel p(x+1,y). 
     
     
       13. The device of  claim 5 , wherein the one or more processors are configured to compute the first filtered pixel value based on the replicated values in which a replicated value for a pixel p(x,y+2) is replicated from a value of the first pixel p(x,y). 
     
     
       14. The device of  claim 5 , wherein the one or more processors are configured to compute the first filtered pixel value based on the replicated values in which a replicated value for a pixel p(x,y+3) is replicated from a value of the first pixel p(x,y). 
     
     
       15. A method comprising:
 determining filter coefficients for a two-dimensional (2D) finite impulse response (FIR) filter; 
 applying the 2D FIR filter using the determined filter coefficients to a first pixel p(x,y) at a first virtual boundary of a reconstructed coding unit, wherein x is a coordinate on an x-axis of the reconstructed coding unit and y is a coordinate on a y-axis of the reconstructed coding unit; and 
 computing a first filtered pixel value based on the filtered first pixel p(x,y) and replicated values in which:
 a replicated value for a pixel p(x,y+2) is replicated from a value of the first pixel p(x,y); 
 a replicated value for a pixel p(x,y+3) is replicated from a value of the first pixel p(x,y); 
 a replicated value for a pixel p(x−1,y−1) is replicated from a value of a pixel p(x−1,y); and 
 a replicated value for a pixel p(x+1,y−1) is replicated from a value of a pixel p(x+1,y). 
 
 
     
     
       16. The method of  claim 15 , wherein determining filter coefficients comprises decoding the filter coefficients from an encoded bit stream. 
     
     
       17. The method of  claim 15 , wherein applying the 2D FIR filter to the first pixel comprises using filter coefficients determined for a neighboring largest coding unit (LCU) of the reconstructed coding unit to compute the first filtered pixel value. 
     
     
       18. The method of  claim 15 , wherein the 2D FIR filter is a ten-tap FIR filter with a vertical size of seven and a horizontal size of nine. 
     
     
       19. The method of  claim 1 , wherein computing the first filtered pixel value is based on the replicated values in which a replicated value for a pixel p(x+1,y−1) is replicated from a value of a pixel p(x+1,y). 
     
     
       20. The device of  claim 5 , wherein the one or more processors are configured to compute the first filtered pixel value based on the replicated values in which a replicated value for a pixel p(x+1,y−1) is replicated from a value of a pixel p(x+1,y).